Among contemporary drilling tools, the drilling tool for performing trepanning, to leave the central columnar portion of a material, generally has low cutting energy consumption and a small thrust at the time of cutting, as compared to other types of cutting tools for cutting into entire portions of pieces, such as those used in cutting a hole. For this reason, only limited machining power can be used effectively, and drilling with higher efficiency can be performed by the same cutting force as in other types of drilling tools. In addition to this advantage, the core portion which is removed can be reused.
However, such a trepan drilling tool comprises a cylindrical cutting tool which does not have a portion corresponding to the center of rotation, so that both chatter vibration and swinging tend to occur during cutting, particularly at the start of cutting. To avoid such problems in the conventional trepan drilling tool, a guide pad is mounted around the tool. Alternatively, a centering drill is mounted at the center of the tool, to reduce the chatter and swinging at the start of drilling, and a guide pad is mounted to guide the tool after the leading part has cut into the material.
In the conventional trepan drilling tool, a guide pad and a guide bush must be mounted, and these members must be adjusted before drilling. Furthermore, a center hole must be predrilled. As a result, the predrilling time becomes longer than the drilling time, so that the entire drilling operation becomes time-consuming and extremely inefficient.
Since the present invention has been contrived to eliminate the drawbacks described above, its primary object is to provide a trepan drilling tool which has good cutting force balance and reduces chatter vibration and swinging, both at the start of drilling and after the leading part has cut into a workpiece.